1. Field of the Invention
The present invention relates to a method for recirculating a high-temperature etching solution, particularly for use in a precise wet etching of a wafer for a semiconductor device.
2. Discussion of Background
In a semiconductor field, recently, with increases in performance and integration of a semiconductor device, it has been required to provide a satisfactory precise etching of a film of silicon oxide (which will be referred to as a film of SiO.sub.2 hereinafter) and a film of silicon nitride (which will be referred to as a film of Si.sub.4 N.sub.3 hereinafter) as in LSIs of so-called MNOS and CMOS types.
In such etching, a reduction in thickness of a gate insulating film is an important point. There is a need of a precise etching treatment in which only the film of Si.sub.4 N.sub.3 of the films of SiO.sub.2 and Si.sub.4 N.sub.3 formed on a silicon substrate is uniformly etched as much as possible, while leaving the film of SiO.sub.2.
With a semiconductor device of 64 K bits, the film of SiO.sub.2 may be relatively thick (e.g., 2,000 .ANG.). With a semiconductor device of 1 M bit, however, the thickness of the film of SiO.sub.2 is decreased (e.g., 1,000 .ANG.) and hence, if the film of Si.sub.4 N.sub.3 is intended to be etched to a thinner level, e.g., by 200 .ANG., even the film of SiO.sub.2 may be etched inevitably.
When the etching cannot be uniformly conducted, or when the concentration and/or temperature of the etching solution are or is higher, there is a problem of a ratio of the etched amount of the film of Si.sub.4 N.sub.3 to the etched amount of the film of SiO.sub.2, i.e, a selection ratio.
In general, a phosphoric acid solution is used as an etching solution for these films. However, if the temperature of this phosphoric acid is raised, the etched amount may be increased, and with a variation in temperature of the phosphoric acid solution, the selection ratio may be also varied.
Such a relationship is as illustrated in FIG. 2.
Therefore, it is necessary to properly select conditions such as the temperature and concentration of the etching solution, the etched amount and the selection ratio depending upon the type of intended semiconductor device, and a wider range of such selection are desired.
To meet such demands, it is a conventional practice to employ an etching method using an apparatus shown in FIG. 3.
FIG. 3 illustrates one example of apparatus used to carry out a conventional method for etching a film of Si.sub.4 N.sub.3 on a wafer having films of SiO.sub.2 and Si.sub.4 N.sub.3 on a silicon substrate by use of a high-temperature phosphoric acid solution as an etching solution.
In this conventional method, a wafer 18 held by a wafer holder 19 is immersed into an etching solution 1 consisting of a phosphoric solution having a high temperature of 150.degree. C. to 180.degree. C. and contained in an etching bath 2 having a primary heater 7, and the wafer holder is supported externally through a hole in a lid 20. An N.sub.2 gas inlet pipe 22 is mounted at a bottom of the etching bath 2. An N.sub.2 gas is injected through the inlet pipe 22 and bubbled through the solution to stir the latter, while at the same time, manually swinging the wafer holder 19, thus performing the etching.
Water in the phosphoric acid solution is evaporated by heating, resulting in an increased concentration of the solution and hence, in order to maintain the concentration thereof constant, pure water as a replenishing water may be manually intermittently added or continuously dropped through a pipe 21.
The adjustment of the temperature of the etching solution 1 is conducted by a temperature controller 15.
In this method, if the etching ability is reduced while the etching is being repeated, the batch of such etching is completed, and the solution in the bath is discarded. Then, a new phosphoric acid solution adjusted in concentration is placed into the bath to start the subsequent batch of etching.
In the above conventional etching method, however, it is difficult to satisfactorily control the etching for excellent uniformity and selectivity when a presice etching is required.
More specifically, in this method, because the replenishing water is manually added directly into the etching bath in order to keep the concentration of the phosphoric acid solution constant, it is difficult to maintain the solution concentration in a very narrow given range, even if the etching solution is stirred by bubbling the N2 gas while swinging the wafer holder.
For example, when the replenishing water is added, the concentration and the temperature are dropped in an initial state of addition and then gradually raised On the other hand, when the addition is performed by dropping, the water at a local place at which the replenishing water has been dropped is boiled to cause a reduction in temperature at such place, resulting in non-uniform concentration and temperature in the etching bath to produce an unevenness in etching on the surface of the wafer.
Another problem in this method is that there is a fear of a mechanical trouble resulting in a complicated operation, such as a foreign matter produced due to &he contact of the wafer holder with a peripheral edge of the hole in the lid because the wafer holder is manually swung.
In addition, the stirring by bubbling of the N.sub.2 gas is accompanied by the following disadvantages: Frequently, the bubbles of N.sub.2 gas may be deposited onto or stagnated on the surface of the wafer, causing the etching thereat to be locally delayed. Moreover, the supplying of the N.sub.2 gas may increase the chance of inclusion of a foregin matter, and the N.sub.2 gas is discharged out of the etching bath and then exhausted outside the system, resulting in a uselessness thereof to cause an uneconomy.
Therefore, with the conventional method, it is difficult to provide the precise etchability for very thin films of SiO.sub.2 and Si.sub.4 N.sub.3, resulting in a lower yield of products. Further, it is impossible to increase the etching temperature and concentration and for this reason, the life of the etching solution may be shortened and consequently, the frequency of replacement of the etching solution may be, of course, increased, leading to a lower productivity.